Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing sides of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Metal-based active materials can often be desirable for use in flow batteries and other electrochemical energy storage systems. Although non-ligated metal ions (e.g., dissolved salts of a redox-active metal) can be used as an active material, it can often be more desirable to utilize coordination complexes for this purpose. As used herein, the terms “coordination complex, “coordination compound,” “metal-ligand complex,” or simply “complex” synonymously refer to a compound having at least one covalent bond formed between a metal center and a donor ligand. The metal center can cycle between an oxidized form and a reduced form in an electrolyte solution, where the oxidized and reduced forms of the metal center represent states of full charge or full discharge depending upon the particular half-cell in which the coordination complex is present.
Metal catecholate complexes can be particularly desirable active materials, since they are relatively stable complexes, have relatively good solubility in aqueous media, and can provide flow batteries having efficient operating characteristics. In some instances, metal catecholate complexes containing only unsubstituted catecholate ligands can be suitable for use within flow batteries. In other cases, substituted catechol compounds having solubilizing groups thereon can improve the aqueous solubility of coordination complexes where they are present. The syntheses of such substituted catechol compounds can frequently proceed from catechol itself (i.e., 1,2-dihydroxybenzene). Although catechol is a relatively inexpensive commodity chemical, a significant amount of hydroquinone byproduct is frequently co-produced in commercial catechol syntheses. While hydroquinone can be separated from catechol prior to incorporation of the latter in coordination complexes, the hydroquinone byproduct represents a significant feedstock waste in terms of atom economy. Moreover, the hydroquinone byproduct presents a substantial waste disposal issue when taking into account the multi-ton quantities of active materials that are anticipated to be needed in support of commercial flow battery applications. At present, the hydroquinone byproduct has no significant use in the flow battery industry.
In view of the foregoing, processes for converting a hydroquinone byproduct into a higher-value material, particularly a material of relevance to the flow battery industry, would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.